The menstrual cycle

Summary

The menstrual cycle is a 20-35 day process, averaging 28 days, that involves the ovaries and endometrium to facilitate reproduction. It encompasses the ovarian and uterine cycles, with ovulation typically at day 14. The cycle is regulated by hormones from the hypothalamus and pituitary gland, leading to the maturation of ovarian follicles and the thickening of the endometrium. After ovulation, the corpus luteum forms, secreting progesterone to prepare the endometrium for potential implantation. If fertilization doesn't occur, the cycle ends with menstruation, beginning a new cycle.

Takeaways

• 🔬 The menstrual cycle is a regular process involving the ovaries and endometrium that facilitates reproduction.
• 🌀 It consists of two synchronized processes: the ovarian cycle, focusing on follicle development and ovulation, and the uterine cycle, focusing on the endometrium's response to ovarian activity.
• 👧 Menarche marks the onset of the first menstrual period during early adolescence, and the cycle recurs monthly until menopause.
• ⏳ The menstrual cycle's length can vary from 20 to 35 days, averaging 28 days, with ovulation typically occurring 14 days before the start of menstruation.
• 📅 Each cycle begins on the first day of menstruation, with the preovulatory phase leading up to ovulation and the postovulatory phase following it.
• 🧠 The hypothalamus and pituitary gland control the menstrual cycle through the release of hormones like GnRH, FSH, and LH.
• 🌱 The ovarian follicular phase involves the maturation of ovarian follicles, with one becoming dominant and eventually undergoing ovulation.
• 🚀 A surge in FSH and LH, triggered by high estrogen levels, induces ovulation, releasing the oocyte from the dominant follicle.
• 🛡️ After ovulation, the ovarian follicle transforms into the corpus luteum, which secretes progesterone, becoming the dominant hormone of the luteal phase.
• 🌿 The endometrium undergoes the menstrual phase, proliferative phase, and secretory phase in response to hormonal changes throughout the cycle.
• 🌹 Progesterone prepares the endometrium for potential fertilization and implantation, and its decline signals the beginning of a new menstrual cycle.

Q & A

What is the menstrual cycle?

The menstrual cycle refers to the regular changes in the ovaries and the endometrium that prepare the body for reproduction. It involves two interconnected processes: the ovarian cycle and the uterine or endometrial cycle.

What is the endometrium and what are its layers?

The endometrium is the tissue lining the inside of the uterus, consisting of a functional layer that is shed during menstruation and a basal layer that supports the functional layer.

What are the two processes that make up the menstrual cycle?

The two processes are the ovarian cycle, which involves the development of ovarian follicles and ovulation, and the uterine or endometrial cycle, which involves the thickening and shedding of the functional endometrium in response to ovarian activity.

What is menarche?

Menarche is the onset of the first menstrual period, typically occurring during early adolescence as part of puberty.

How long does the average menstrual cycle last?

The average menstrual cycle lasts about 28 days, but it can vary from 20 to 35 days.

When does ovulation typically occur in the menstrual cycle?

Ovulation usually occurs 14 days before the first day of the next menstrual period, which is about day 14 of an average 28-day cycle.

What are the two phases of the menstrual cycle and what do they correspond to in the ovaries and endometrium?

The two phases are the preovulatory phase, which includes the ovarian follicular phase and the endometrial proliferative phase, and the postovulatory phase, which includes the ovarian luteal phase and the endometrial secretory phase.

What role do the hypothalamus and pituitary gland play in the menstrual cycle?

The hypothalamus and pituitary gland control the menstrual cycle by regulating the release of gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH), which in turn control the maturation of ovarian follicles and the endometrial changes.

What is the significance of the dominant follicle in the preovulatory phase?

The dominant follicle is the one that continues to grow and eventually undergoes ovulation due to having the most follicle-stimulating hormone receptors, outcompeting other developing follicles.

What changes occur in the endometrium during the proliferative phase?

During the proliferative phase, the endometrium thickens, endometrial glands grow, and spiral arteries emerge under the influence of estrogen, preparing the lining for potential implantation of a fertilized egg.

What happens after ovulation and how does it affect the endometrium?

After ovulation, the remnants of the ovarian follicle form the corpus luteum, which secretes progesterone and estrogen, making the endometrium more receptive for implantation. This leads to the secretory phase of the endometrial cycle.

What is the role of progesterone during the luteal phase?

Progesterone, secreted by the corpus luteum, acts as a negative feedback signal to the pituitary, decreasing the release of FSH and LH, and becomes the dominant hormone during the luteal phase, preparing the endometrium for potential implantation.

What triggers the beginning of a new menstrual cycle?

The collapse of the spiral arteries and the shedding of the functional layer of the endometrium, which occurs when progesterone levels decrease, triggers menstruation and the beginning of a new menstrual cycle.

Outlines

00:00

🌱 Menstrual Cycle and Ovarian Follicular Phase

The menstrual cycle is a biological process involving the ovaries and endometrium that facilitates reproduction. It encompasses the ovarian and endometrial cycles, with the former focusing on follicle development and ovulation, and the latter on the endometrium's response to hormonal changes. Menarche marks the first menstrual period during adolescence, and the cycle recurs monthly until menopause. The average cycle is 28 days, with ovulation typically occurring 14 days before menstruation begins. The preovulatory phase includes the menstrual and proliferative phases of the endometrium and is controlled by the hypothalamus and pituitary gland, which regulate the maturation of ovarian follicles through the secretion of GnRH, FSH, and LH. The dominant follicle is determined by the follicle's sensitivity to FSH, leading to increased estrogen production and a positive feedback loop that triggers ovulation.

05:01

🌼 Ovulation and the Uterine Proliferative Phase

The dominant follicle's surge in estrogen levels heightens the pituitary's response to GnRH, leading to a significant release of FSH and LH just before ovulation. This hormonal surge is crucial for the rupture of the ovarian follicle and the release of the oocyte. Concurrently, the uterus prepares for potential pregnancy by shedding the old endometrial lining during menstruation and then entering the proliferative phase. High estrogen levels during this phase promote endometrial thickening, gland growth, and spiral artery development, creating a conducive environment for sperm and optimizing the chance of fertilization, which peaks around day 11 to 15 of a 28-day cycle.

10:02

🌕 Post-Ovulation Changes and the Luteal Phase

After ovulation, the ovarian follicle remnants form the corpus luteum, which secretes hormones under the influence of LH. The luteal phase is characterized by a shift from estrogen to progesterone dominance, with the corpus luteum converting androstenedione to 17β-estradiol and increasing progesterone production. Progesterone and inhibin act as negative feedback signals, reducing FSH and LH secretion and leading to a decline in estrogen levels. The endometrium enters the secretory phase, with spiral arteries coiling and uterine glands increasing mucus secretion to prepare for implantation. If fertilization does not occur, the corpus luteum degenerates, leading to a drop in hormone levels, spiral artery collapse, and the onset of a new menstrual cycle.

📊 Menstrual Cycle Overview and Phases

The menstrual cycle, averaging 28 days, begins with menstruation and is divided into the follicular and luteal phases. The follicular phase involves the development of the ovarian follicles and culminates in ovulation around day 14 due to a surge in LH. The luteal phase, dominated by progesterone, precedes menstruation by 14 days and is crucial for potential implantation. The uterus undergoes the menstrual phase, proliferative phase, and secretory phase in sync with ovarian activity, preparing for fertilization and, if unsuccessful, shedding the lining to initiate a new cycle.

Keywords

💡Menstrual cycle

The menstrual cycle is a biological process that prepares a female's body for potential pregnancy by undergoing a series of hormonal changes. It is central to the video's theme, as it describes the regular changes in the ovaries and the endometrium. The script explains that this cycle can vary in length but averages around 28 days, beginning with menstruation and including phases such as the follicular and luteal phases.

💡Endometrium

The endometrium is the tissue lining the inside of the uterus, consisting of a functional layer that is shed during menstruation and a basal layer. It plays a crucial role in the menstrual cycle, as it thickens in response to ovarian activity to prepare for potential implantation of a fertilized egg. The script details how the endometrium goes through phases of shedding and thickening, illustrating its importance in reproduction.

💡Menarche

Menarche is the term used to describe the onset of a female's first menstrual period, marking the beginning of her reproductive years during puberty. It is mentioned in the script as a significant milestone in a female's life, after which the menstrual cycle recurs monthly until menopause.

💡Ovulation

Ovulation is the release of an oocyte (egg) from the ovary, typically occurring 14 days before the start of the next menstrual period. It is a pivotal event in the menstrual cycle, as it represents the opportunity for fertilization. The script explains that ovulation is triggered by a surge in luteinizing hormone, which is induced by rising estrogen levels from the dominant follicle.

💡Ovarian cycle

The ovarian cycle is one of the two interconnected processes of the menstrual cycle, focusing on the development of ovarian follicles and ovulation. It is integral to the video's narrative, as it outlines the stages of follicle maturation and the hormonal interplay leading to ovulation, which is essential for reproduction.

💡Uterine or endometrial cycle

The uterine or endometrial cycle refers to the changes in the endometrium in response to ovarian activity. It is closely related to the ovarian cycle, as it involves the thickening and shedding of the endometrium. The script describes how this cycle is synchronized with the ovarian cycle, with the endometrium preparing for potential implantation during the proliferative phase and becoming receptive during the secretory phase.

💡Follicular phase

The follicular phase is the first half of the ovarian cycle, leading up to ovulation. It is characterized by the maturation of ovarian follicles and the production of estrogen. The script provides a detailed account of the hormonal changes during this phase, including the development of the dominant follicle and the surge in luteinizing hormone that triggers ovulation.

💡Luteal phase

The luteal phase follows ovulation and is the second half of the ovarian cycle. It is marked by the formation of the corpus luteum and the production of progesterone, which prepares the endometrium for potential implantation. The script explains how the luteal phase is crucial for maintaining the endometrium should fertilization occur.

💡Hypothalamus and pituitary gland

The hypothalamus and pituitary gland are key components of the hormonal regulation of the menstrual cycle. They act as the 'masterminds' of reproduction, with the hypothalamus releasing gonadotropin-releasing hormone (GnRH) that stimulates the pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The script describes their roles in controlling the maturation of ovarian follicles and the timing of ovulation.

💡Corpus luteum

The corpus luteum is the structure formed from the remnants of the ovarian follicle after ovulation. It secretes progesterone and estrogen, which are essential for maintaining the endometrium for potential pregnancy. The script explains the function of the corpus luteum in the luteal phase and its importance in the menstrual cycle.

💡Progesterone

Progesterone is a hormone produced primarily by the corpus luteum after ovulation. It is vital for preparing the endometrium for implantation and maintaining early pregnancy. The script describes how progesterone becomes the dominant hormone in the luteal phase and its role in signaling the occurrence of ovulation and making the endometrium receptive to a fertilized egg.

Highlights

The menstrual cycle involves regular changes in the ovaries and endometrium for reproduction, consisting of the ovarian and uterine cycles.

Menarche marks the onset of the first menstrual period during early adolescence as part of puberty.

The menstrual cycle recurs monthly, pausing only during pregnancy, until menopause when ovarian function declines.

The average menstrual cycle duration is 28 days, varying from 20 to 35 days.

Ovulation usually occurs 14 days before the start of the next menstrual cycle.

The ovarian follicular phase leads up to ovulation, while the luteal phase follows ovulation.

The hypothalamus and pituitary gland control the menstrual cycle through the secretion of GnRH, FSH, and LH.

Follicles grow and compete for ovulation during the follicular phase, with the dominant follicle eventually undergoing ovulation.

Estrogen levels rise during the follicular phase, acting as a negative feedback signal to the pituitary to regulate FSH secretion.

A surge in FSH and LH just before ovulation is responsible for inducing the release of the oocyte.

The uterus prepares the endometrium for potential pregnancy during the menstrual and proliferative phases.

High estrogen levels during the proliferative phase stimulate endometrial thickening and gland growth.

The cervical mucus becomes more hospitable to sperm around days 11-15 of a 28-day cycle, optimizing chances of fertilization.

After ovulation, the ovarian follicle remnant becomes the corpus luteum, secreting progesterone and estrogen.

Progesterone becomes the dominant hormone during the luteal phase, signaling ovulation and preparing the endometrium for implantation.

The secretory phase of the endometrial cycle involves spiral artery growth, uterine gland secretion, and a decline in estrogen levels.

The corpus luteum degenerates into the corpus albicans, leading to a decrease in estrogen and progesterone levels, and the start of a new menstrual cycle.

Transcripts

The menstrual cycle refers to the regular changes in the activity of the ovaries and

the endometrium that make reproduction possible. The endometrium is the layer of tissue lining

the inside of the uterus. This lining consists of a functional layer, which is subject to

hormonal changes and is shed during menstruation, and a thin basal layer which feeds the overlying

functional layer. The menstrual cycle actually consists of two interconnected and synchronized

processes: the ovarian cycle, which centers on the development of the ovarian follicles

and ovulation, and the uterine or endometrial cycle, which centers on the way in which the

functional endometrium thickens and sheds in response to ovarian activity.

Menarche, which refers to the onset of the first menstrual period, usually occurs during

early adolescence as part of puberty. Following menarche, the menstrual cycle recurs on a

monthly basis, pausing only during pregnancy, until a person reaches menopause, when her

ovarian function declines and she stops having menstrual periods. The monthly menstrual cycle

can vary in duration from 20 to 35 days, with an average of 28 days. Each menstrual cycle

begins on the first day of menstruation, and this is referred to as day one of the cycle.

Ovulation, or the release of the oocyte from the ovary, usually occurs 14 days before the

first day of menstruation (i.e., 14 days before the next cycle begins). So, for an average

28-day menstrual cycle, this means that there are usually 14 days leading up to ovulation

(i.e., the preovulatory phase) and 14 days following ovulation (i.e., the postovulatory

phase). During these two phases, the ovaries and the endometrium each undergo their own

set of changes, which are separate but related. As a result, each phase of the menstrual cycle

has two different names to describe these two different parallel processes. For the

ovary, the two weeks leading up to ovulation is called the ovarian follicular phase, and

this corresponds to the menstrual and proliferative phases of the endometrium. Similarly, the

two weeks following ovulation is referred to as the ovarian luteal phase, which also

corresponds to the secretory phase of the endometrium.

So, let’s first focus on the preovulatory period, starting with the ovarian follicular

phase. This phase starts on the first day of menstruation and represents weeks one and

two of a four-week cycle. The whole menstrual cycle is controlled by the hypothalamus and

the pituitary gland, which are like the masterminds of reproduction. The hypothalamus is a part

of the brain that secretes gonadotropin-releasing hormone, or GnRH, which causes the nearby

anterior pituitary gland to release follicle stimulating hormone, or FSH, and luteinizing

hormone, or LH. Before puberty, the gonadotropin-releasing hormone is released at a steady rate, but

once puberty hits, the gonadotropin-releasing hormone is released in pulses, sometimes more

and sometimes less. The frequency and magnitude of the gonadotropin-releasing hormone pulses

determine how much follicle stimulating hormone and luteinizing hormone will be produced by

the pituitary. These pituitary hormones control the maturation of the ovarian follicles, each

of which is initially made up of an immature sex cell, or primary oocyte, surrounded by

layers of theca and granulosa cells, the hormone-secreting cells of the ovary.

Over the course of the follicular phase, these oocyte-containing groups of cells, or follicles,

grow and compete for a chance at ovulation. During the first ten days, theca cells develop

receptors and bind luteinizing hormone, and in response secrete large amounts of the hormone

androstenedione, an androgen hormone. Similarly, granulosa cells develop receptors and bind

follicle stimulating hormone, and in response produce the enzyme aromatase. Aromatase converts

androstenedione from the theca cells into 17β-estradiol, which is a member of the estrogen

family. During days 10 through 14 of this phase, granulosa cells also begin to develop

luteinizing hormone receptors, in addition to the follicle stimulating hormone receptors

they already have.

As the follicles grow and estrogen is released into the bloodstream, increased estrogen levels

act as a negative feedback signal, telling the pituitary to secrete less follicle stimulating

hormone. As a result of decreased follicle stimulating hormone production, some of the

developing follicles in the ovary will stop growing, regress and die off. The follicle

that has the most follicle stimulating hormone receptors, however, will continue to grow,

becoming the dominant follicle that will eventually undergo ovulation. This dominant follicle

continues to secrete estrogen, and the rising estrogen levels make the pituitary more responsive

to the pulsatile action of gonadotropin-releasing hormone from the hypothalamus.

As blood estrogen levels start to steadily climb higher and higher, the estrogen from

the dominant follicle now becomes a positive feedback signal – that is, it makes the

pituitary secrete a whole lot of follicle stimulating hormone and luteinizing hormone

in response to gonadotropin-releasing hormone. This surge of follicle stimulating hormone

and luteinizing hormone usually happens a day or two before ovulation and is responsible

for stimulating the rupture of the ovarian follicle and the release of the oocyte. You

can think of it this way: for most of the follicular phase, the pituitary saves its

energy, then when it senses that the dominant follicle ready for release, the pituitary

uses all its energy to secrete enough follicle stimulating hormone and luteinizing hormone

to induce ovulation.

While the ovary is busy preparing an egg for ovulation, the uterus, meanwhile, is preparing

the endometrium for implantation and maintenance of pregnancy. This process begins with the

menstrual phase, which is when the old endometrial lining, or functional layer, from the previous

cycle is shed and eliminated through the vagina, producing the bleeding pattern known as the

menstrual period.

The menstrual phase lasts an average of five days and is followed by the proliferative

phase, during which high estrogen levels stimulate thickening of the endometrium, growth of endometrial

glands, and emergence of spiral arteries, which grow a little under the influence of

estrogen, from the basal layer to feed the growing functional endometrium. Rising estrogen

levels also help change the consistency of the cervical mucus, making it more hospitable

to incoming sperm. The combined effects of this spike in estrogen on the uterus and cervix

help to optimize the chance of fertilization, which is highest between day 11 and day 15

of an average 28-day cycle.

Following ovulation, the remnant of the ovarian follicle becomes the corpus luteum, which

is made up of luteinized theca and granulosa cells, meaning that these cells have been

exposed to the high luteinizing hormone levels that occur just before ovulation. Luteinized

theca cells keep secreting androstenedione, and the luteinized granulosa cells keep converting

it to 17β-estradiol, as before. However, luteinized granulosa cells also respond to

the low luteinizing hormone concentrations that are present after ovulation by increasing

the activity of cholesterol side-chain cleavage enzyme, or P450scc for short. This enzyme

converts more cholesterol to pregnenolone, a progesterone precursor. So luteinized granulosa

cells secrete more progesterone than estrogen during the luteal phase. Progesterone acts

as a negative feedback signal on the pituitary, decreasing release of follicle stimulating

hormone and luteinizing hormone. At the same time, luteinized granulosa cells begin secreting

inhibin, which similarly inhibits the pituitary gland from making follicle stimulating hormone.

Both of these processes result in a decline in estrogen levels, meaning that progesterone

becomes the dominant hormone present during this phase of the cycle. Together with the

decreased level of estrogen, the rising progesterone level signals that ovulation has occurred

and helps make the endometrium receptive to the implantation of a fertilized gamete.

Under the influence of progesterone, the uterus enters into the secretory phase of the endometrial

cycle. During this time spiral arteries grow the most and become coiled, and the uterine

glands begin to secrete more mucus.

After day 15 of the cycle, the optimal window for fertilization begins to close. The cervical

mucus starts to thicken and becomes less hospitable to the sperm. Over time, the corpus luteum

gradually degenerates into the nonfunctional corpus albicans. The corpus albicans doesn’t

make hormones, so estrogen and progesterone levels slowly decrease. When progesterone

reaches its lowest level, the spiral arteries collapse, and the functional layer of the

endometrium prepares to shed through menstruation. This shedding marks the beginning of a new

menstrual cycle and another opportunity for fertilization.

All right, so as a quick recap - the menstrual cycle begins on the first day of menstruation.

For an average 28-day menstrual cycle, the changes which occur in the ovary during the

first 14 days are called the follicular phase. Ovulation usually occurs at day 14, as a result

of the estrogen-induced surge in luteinizing hormone. The last 14 days of the cycle are

the luteal phase, during which progesterone becomes the dominant hormone. While the length

of the follicular phase can vary, the luteal phase almost always precedes the onset of

menses by 14 days. The uterus also goes through its own set of changes. During the first 14

days of the cycle, the endometrium goes through the menstrual phase and the proliferative

phase, and during the last 14 days it goes through the secretory phase.